Embodiments of the invention relate generally to x-ray tubes and, more particularly, to an apparatus for forming an expansion joint and a method of constructing same.
Computed tomography (CT) X-ray imaging systems typically include an x-ray tube, a detector, and a gantry assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector converts the received radiation to electrical signals and then transmits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner.
A typical x-ray tube includes a cathode that provides a focused high energy electron beam that is accelerated across a cathode-to-anode vacuum gap and produces x-rays upon impact with an active material or target provided. Because of the high temperatures generated when the electron beam strikes the target, typically the target assembly is rotated at high rotational speed for purposes of cooling the target. Components of the x-ray tube are placed in a ultra-high vacuum which is maintained by a frame that is typically made of metal or glass.
The x-ray tube also includes a rotating subsystem that rotates the target for the purpose of distributing the heat generated at a focal spot on the target. The rotating subsystem is typically rotated by an induction motor having a cylindrical rotor built into an axle that supports a disc-shaped target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating subsystem assembly is driven by the stator. Typically, the target is supported by a bearing assembly in a cantilever type arrangement. The bearing assembly is comprised of a front inner/outer bearing race and a rear inner/outer bearing race, ball bearings, and a shaft extends therefrom to support the target. The bearing assembly is axially anchored on one end such that, in a typical design the shaft supporting the target is able to expand and contract freely during operation and as a result of the extreme temperatures experienced during operation.
In recent years, it has been desired within the CT industry to increase gantry speeds to 0.4 seconds gantry rotation and faster. As the industry drives to faster gantry speeds, the mechanical loading on x-ray tubes has increased as well. Generally the mechanical loading on an x-ray tube increases as the square of the gantry rotational speed, thus increased gantry speeds have lead to enormous g-loading on the x-ray tube and particularly on the target. Accordingly, the mechanical loading on the support bearing assembly of the target has increased dramatically as well.
As such and in order to accommodate the increased gantry speeds, in some known designs the target is supported by a single shaft, but a flange is incorporated that enables the target to be positioned between the front and rear races of the bearing assembly (sometimes referred to as a reentrant design). This positions the target proximate to both the front and rear races, and in some known designs the target is positioned such the center of gravity of the rotating subsystem is centered between the front and rear races, which enables equal load sharing between the front and rear races. In other known designs a spiral groove bearing (SGB) may be incorporated, in lieu of ball bearing-based bearing assemblies, that provides a much broader distribution of stress over a low vapor fluid (liquid metal fluid) that is positioned between inner and outer components that rotate with respect to one another under a relatively small gaps, approximately 15 microns in one known embodiment. One known fluid in a SGB is gallium.
However, it has been desired in recent years to increase gantry speeds yet more, to 0.25 gantry speeds and faster. As such, known bearing designs may fail either catastrophically or through a shortened life due to wear in these increased g-load conditions. Increased gantry speeds can also cause relatively large mechanical deflections of the target support structure (shaft, bearing & target) that can cause focal spot motion or other sources of image quality problems. Thus, in order to enable operation in 0.25 seconds gantry speed and faster, recent x-ray tube designs have included a shaft that is supported on both axial sides of the target. That is, the rotatable shaft to which the target and rotor are attached may include a bearing stationary support (ball-bearing or SGB, as examples) that is hard connected to a plate or other support structure of the x-ray tube. In other words, in order to accommodate the dramatically increased loads for gantry speeds of 0.25 seconds or greater, it is desirable to support the target with supports that are positioned on both sides of the target, providing a ‘straddle’ support that significantly reduces the concentrated load and deflection on the bearing and removes the cantilever affect of a cantilever-mounted target.
However, in order to do so (that is, to provide the second support) the second support is typically hard mounted to the frame of the x-ray tube. As such, the support mechanically constrains the shaft axially on its second end as well, precluding the shaft from being able to freely expand and contract during operation and during other heating and cooling events.
Typically the components of the x-ray tube are made of different materials for different reasons. For instance, the shaft itself is often made of molybdenum (because of its ability to sustain high temperatures during operation), while the support plate and frame to which the shaft is attached is typically made of a far less expensive material such as stainless steel. Because of the mismatched coefficients of thermal expansion (CTE) and weldability, as examples, kovar is typically included as an interim material between the shaft and the support plate. The frame itself, attached to the support plate and used to enclose the target, rotor, and other components, may be made of 304L, for example. As such, for a variety of reasons that include but are not limited to material cost, processing and machining expense, performance (i.e., high temperature operation), and weldability, a variety of materials is typically used to form the shaft, plate, frame, and other components that support and enclose the target. Because each material has its own axial length, CTE, overall operating temperature and because the shaft is hard mounted at both ends, differential thermal growth can induce high stresses at interfaces (in welds and brazes) and component parts for the variety of thermal conditions experienced.
Because of the very high processing and operating temperatures in x-ray tubes, x-ray tube components such as the target and its supporting shaft are made with refractory metals such as Molybdenum. Molybdenum is characterized by a low coefficient of thermal expansion (CTE) compared to ferrous metals. The supporting shaft is itself supported and enclosed by the vacuum frame and a support plate, which are generally made from an austenitic stainless steel (304), which has a CTE that is approximately three times that of Molybdenum or alloys thereof. Thus, although the target, the supporting shaft, and the vacuum frame and the support plate may not be made of these specific materials, they are nevertheless typically made of materials in which a large CTE difference occurs at interfaces. The differences of material CTEs and the overall length of the relatively large parts can cause large differential thermal growth between the shaft and its linked components. When combined also with a typically relatively high component stiffness for load capability and deflection control, high internal stresses can be induced at the component interfaces that may include weld and braze joints. The weld or braze joints therefore can present modes of failure that may include a vacuum leak at the joint or a mechanical joint failure that can even lead to a catastrophic tube failure.
As such, one known method of reducing stresses in the components and interfaces is to selectively design the components such that the changes in lengths, that result from temperature changes, balance one another (zero differential thermal growth). That is, based on a thermal model, temperature distributions of the component parts may be predicted and then materials and component related geometric length can be selected such that they balance the changes in lengths that can occur as a result of the predicted temperature distributions. For instance, during operation the center shaft made of Molybdenum, although having a lower expansion coefficient than the 304L frame material, the center shaft may nevertheless expand more than the frame because of the much higher temperature at which it the center shaft operates. Thus, in this example, in order to counteract the effect, a material having a higher CTE than 304 L can be included in a portion of the frame (reentrant rotor) such that the parts expand the same amount when the component parts reach their steady state operating temperature. Also nickel based alloys such as Ni42 with lower CTE than SS304L or a hybrid frame assembly made of ceramic, kovar, or nickel base alloys could be used in the frame construction to reduce the overall component thermal growth.
However, although component parts can be designed that minimize the stresses that result at temperature, not all thermal conditions are the same for the x-ray tube. For instance, x-ray tubes operate at a wide range of steady state or average powers, thus one set of assumed steady state thermal conditions may not suffice to minimize stress in the components when a different steady state occurs. One day may see a lot of high power imaging with a heavy patient load, while on other days only low power scans may be conducted. Further and regardless, while heating and cooling, the components experience transient thermal responses (temperature distributions) that can cause stresses to occur, due to differential dynamic expansion during the transients, that can cause stresses to occur even if the stresses are reduced to near zero when they do reach steady state.
In addition, aside from the extreme temperatures experienced during typical x-ray tube operation, during manufacture the x-ray tube may go through significant temperature excursions during processing such as bakeout and seasoning. As one example, during bakeout the entire x-ray tube (frame, support plate, shaft, etc. . . . ) is brought to a high temperature (approximately over 400° C.). Typically, the x-ray tube is baked in an oven in order to bring all component parts up to sufficient temperature so as to clean all the exposed surfaces and provide longterm high voltage stability. During bakeout the frame in particular experiences a much higher temperature excursion that typically occurs during normal operation in an x-ray tube. As such, even if component parts are designed in order to survive various steady state and transient conditions, bakeout and other processing steps can cause worse differential thermal growth than those under tube operating conditions.
Thus, when both ends of the stationary shaft of the rotating subsystem are hard mounted to the frame, enormous stresses can result at the component interfaces and at the component itself as the overall system heats due to processing or operating thermal condition from room temperature. The stresses can be reduced to an extent by designing components appropriately such that interfaces and component stresses are within design limits for a given set of thermal conditions. However, an x-ray tube can see a wide variety of steady state and transient conditions, as well as different operating conditions. As such, not all possible sets of thermal conditions can be designed for, and component stresses can occur that can lead to fatigue cycling and/or catastrophic component failure.
Accordingly, it would be advantageous to have an x-ray tube having a robust design with joints between components that can maintain an ultra-high vacuum under a wide range of thermal conditions during operation and processing and overcome the aforementioned drawbacks.